Thousands and beyond. To be fair, there's a few obscure optimizations
that will have to come on line as these edge cases get exercised but,
once we're into a few thousands of changesets in a version --- if your
project has more than that in a given year then you are probably out
of control.

Well, determining the patchlevel is an O(n) operation. No matter how
fast it is, it'll be slow if you throw enough revisions at it. See my
other post for an optimization that makes the O(n) part a local operation.